Mechanical switch circuit comprising contact conditioning means

ABSTRACT

Two reed switches are alternately closed by a magnet rotating with a shaft of a water meter or the like which is to be read remotely. A capacitor discharges through the reed switches when the same are closed, increasing the switch current without increasing the external power source current, thereby producing temporary partial fusion of the switch contacts and maximum contact effectiveness. A low-pass filter and a snap action circuit such as a Schmitt trigger are connected to the switches to eliminate the effects of contact bouncing and noise signals from the output of the circuit.

BACKGROUND OF THE INVENTION

The present invention relates to a mechanical switch circuit comprising contact conditioning means which is ideally suited to a system for remote reading of utility meters or the like.

The high cost of personnel for reading utility meters has led to the development of remote reading systems. Taking a water meter as an example, a mechanical switch may be connected to the internal gearing of the meter such as to close each time one liter of water is consumed. The switch is connected through cables to a remote utility station which senses each time the switch is closed.

These systems are coming into widespread use due to their inherent advantages over direct reading. However, several problems have remained heretofore unsolved which have prevented complete acceptance of such systems.

The first problem is the sensitivity of the system to noise produced in the cables connecting the meter switches to the remote utility station. Since these cables must often be quite long, many opportunities for the introduction of electrical noise are present. Noise pulses cause erroneous operation of the sensing means in the utility station and result in overly high readings and overbilling of the utility consumer. The time required for investigation and correction of the erroneous billing may exceed the time required for direct reading of the meter, thus obviating the advantage of the remote reading system.

Another related problem is erroneous operation of the sensing means caused by bouncing of the switch contacts. When a mechanical switch is closed, the contacts bounce against each other as many as several hundred times before mechanical and electrical equilibrium is reached. Each opening and closing of the contacts gives rise to an electrical pulse which can be counted by the remote sensing means. Thus, contact bouncing also produces erroneously high readings and overbilling.

The bouncing phenomenon becomes worse with time due to the deterioration of the switch. A related but opposite phenomenon is the increase in the contact resistance caused by deterioration. In extreme cases the resistance may become so high that some switch closures may not be detectd by the sensing means. The combination of contact bouncing which produces erroneously high readings and increased contact resistance which produces erroneously low readings may result in utility readings which are totally unrelated to the amount of the utility consumed.

The effects of contact bouncing and deterioration may be minimized by passing a large amount of current through the switch contacts. This causes temporary partial fusion of the contact surfaces which maximizes the contact efficiency and rejuvenates the contacts at each closure. Although it is desirable to provide such an effect in a remote utility reading system, such has been heretofore been impractical due to the high power requirement.

SUMMARY OF THE INVENTION

In accordance with the present invention two reed switches are alternately closed by a magnet rotating with a shaft of a water meter or the like which is to be read remotely. A capacitor discharges through the reed switches when the same are closed, increasing the switch current without increasing the external power source current, thereby producing temporary partial fusion of the switch contacts and maximum contact effectiveness. A low-pass filter and a snap action circuit such as a Schmitt trigger are connected to the switches to eliminate the effects of contact bouncing and noise signals from the output of the circuit.

It is an object of the present invention to provide a mechanical switch circuit comprising an improved contact conditioning or debouncing means which is suited for remote reading of utility meters and the like, provides reliable operation and is resistant to the effects of contact bouncing, deterioration and externally produced electrical noise.

It is another object of the present invention to provide a mechanical switch circuit comprising unique means for increasing the current flow through switch contacts without increasing the external power source requirements.

It is another object of the present invention to provide a mechanical switch circuit which automatically rejuvenates switch contacts upon closure thereof and increases the contact effectiveness and service life of the contacts over the prior art.

It is another object of the present invention to provide a generally improved mechanical switch circuit for remote meter reading.

Other objects, together with the foregoing, are attained in the embodiments described in the following description and illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an electrical schematic diagram, partly in block form, of a mechanical switch circuit embodying the present invention;

FIGS. 2a and 2b are graphs illustrating the operation of the present circuit;

FIG. 3 is an electrical schematic diagram of a first embodiment of a snap action circuit;

FIG. 4 is similar to FIG. 3 but shows a second embodiment;

FIGS. 5a and 5e are graphs illustrating the operation of the snap action circuit of FIG. 4; and

FIG. 6 is similar to FIG. 1 but shows a modified embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the mechanical switch circuit of the invention is susceptible of numerous physical embodiments, depending upon the environment and requirements of use, substantial numbers of the herein shown and described embodiments have been made, tested and used, and all have performed in an eminently satisfactory manner.

Referring now to FIG. 1, a mechanical switch circuit embodying the present invention is generally designated by the reference numeral 11 and comprises a D.C. power source which is symbolically shown as a battery 12. A first resistor R₁, second resistor R₂ and first capacitor C₁ are connected in series between first and second ends of the battery 12 or circuit points 13 and 14 respectively in such a manner that the capacitor C₁ is connected between the resistors R₁ and R₂. A second capacitor C₂ is connected at a second end thereof to the point 14. Reed switches RS₁ and RS₂ are connected between a first end of the capacitor C₂ or circuit point 16 and first and second ends of the capacitor C₁ or circuit points 17 and 18 respectively. A magnet assembly 19 is fixed for rotation with a shaft 21 of a water meter or the like (not shown). The reed switches RS₁ and RS₂ are normally open and adapted to be alternately closed by the magnet assembly 19 each time the shaft 21 makes one revolution indicating that a unit amount of utility, such as 1 liter of water, has been consumed. A bypass capacitor C₃ is connected across the battery 12.

A low-pass filter 22 comprises a third resistor R₃ and a fourth capacitor C₄ which are connected in series across the points 16 and 14, the third resistor R₃ and the fourth capacitor C₄ constituting a first stage of the low-pass filter 22. A resistor R₄ and capacitor C₅ connected across the capacitor C₄ constitute a second stage of the low-pass filter 22. An input of a snap action circuit 23 is connected across the capacitor C₅, an output of the snap action circuit 23 constituting an output of the switch circuit 11.

As shown in FIG. 3, the snap action circuit 23 is constituted by a Schmitt trigger comprising first and second cross-coupled inverters I₁ and I₂ respectively. More specifically, the junction of the resistor R₄ and capacitor C₅ of the low-pass filter 22 is connected through a resistor R₅ to an input of the inverter I₁. An output of the inverter I₁ is connected directly to an input of the circuit 11 and is connected through a feedback resistor R₆ to the input of the inverter I₁. The resistor R₆ allows summing of the output of the inverter I₂ with the voltage across the capacitor C₅. The resistor R₅ serves the dual function of the increasing the effective input impedance of the inverter I₁ and allowing summing of the voltage across the capacitor C₅ with the output of the inverter I₂. It is desirable that the input impedance of the snap action circuit 23 be as high as possible for reasons which will become apparent from description hereinbelow.

In operation, the reed switch RS₁ is closed by rotation of the magnet assembly 19 indicating consumption of the utility. The reed switch RS₂ is arranged to be subsequently closed after the reed switch RS₁ is opened upon further rotation of the shaft 21 and magnet assembly 19. Upon closure of the reed switch RS₁ at a time t₁ shown in FIGS. 2a and 2b, the capacitor C₂ begins to be charged through the resistor R₁ and reed switch RS₁. The peak current which flows through the resistor R₁ at the time t₁ has the value V/r₁ is the voltage of the battery 12 and r₁ is the resistance of the resistor R₁. However, prior to closure of the reed switch RS₁ the capacitor C₁ was charged up to the battery voltage V. With the capacitor C₂ initially discharged, the effect of closing the reed switch RS₁ is equivalent to shorting the point 17 to the point 14. Thus, the capacitor C₁ discharges through the resistor R₂ and reed switch RS₁, the peak current flow through the resistor R₂ being V/r₂, where r₂ is the resistance of the resistor R₂.

Since the current which flows through both resistors R₁ and R₂ also flows through the reed switch RS₁, the peak current flow through the reed switch RS₁ at the time t₁ is V/r₁ + V/r₂. If the values of R₁ and R₂ are selected so that r₁ = r₂, the peak current flow through the reed switch RS₁ is twice as much as if the capacitor C₁ were not provided.

The capacitor C₁ provides substantially increased current flow through the reed switch RS₁ without a corresponding increase in the current from the battery 12, and causes temporary fusion of the contacts (not designated) of the reed switch RS₁. This increases the contact effectiveness and rejuvenates the contact surfaces. This is known in the art as the "cohere" or "fritting" effect. Thus, deterioration of the contacts of the reed switch RS₁, bouncing of the contacts and contact resistance are minimized. In a practical installation, the battery 12 is located far from the reed switches RS₁ and RS₂. Increasing the current supply from the battery 12 to provide temporary fusion of the contacts would result in greatly increased I² R losses in the connecting cables and an unacceptable loss of electrical power. However, the unique configuration of the capacitor C₁ increases the current flow through the reed switch RS₁ without increasing the current flow from the battery 12, thereby accomplishing the desired function with substantial economies.

In FIGS. 2a and 2b, the abscissa axes represent time and the ordinate axes represent current I through the reed switch RS₁ or RS₂ (whichever is closed) and voltage E across the low-pass filter 22 respectively. It will be seen that at the time t₁ the current I attains its peak value and decreases exponentially as the capacitor C₂ is charged. The voltage E increases exponentially due to charging of the capacitor C₂ through the resistor R₁ and reed switch RS₁. Since the reed switch RS₁ has mechanical contacts, it is impossible to completely prevent contact bouncing, and in the same time immediately following t₁, FIG. 2a shows the effect of contact bouncing in that the current flow is reduced to zero a number of times as the contacts disengage.

The low-pass filter 22 and snap action circuit 23 serve to condition or debounce the contacts of the reed switches RS₁ and RS₂. These circuits do not affect the switch contacts per se but rather serve to remove the effects of the switch contact bounce from the signal output. The capacitor C₁ serves, in addition to increasing the current flow through the reed switches RS₁ and RS₂, the dual function of suppressing noise appearing on the relatively long signal lines between the reed switches RS₁ and RS₂ and the circuit points 17 and 18. The capacitor C₂ serves to suppress noise appearing on the long signal lines between the reed switches RS₁ and RS₂ and the circuit point 16. The capacitor C₃ serves to suppress noise generated in the battery 12, which is replaced in actual practice by a D.C. power supply (not shown). The value of the resistors R₁ and R₂ and capacitors C₁ and C₂ are selected so that the charging time constant of the capacitor C₂ is long compared to the settling time of the contacts of the reed switches RS₁ and RS₂. Thus, the voltage E across the capacitor C₂ is substantially uneffected by the contact bouncing and increases and decreases smoothly as shown in FIG. 2b. The capacitors C₁, C₄ and C₅ are charged along with the capacitor C₂ and the rising output voltage of the low-pass filter 22 is applied to the input of the snap action circuit 23.

With the voltage applied to the inverter I₁ through the resistor R₅ below the upper trip point of the inverter I₁, the inverter I₁ produces a high output which causes the inverter I₂ to produce a low output. The low output of the inverter I₂ is summed through the resistor R₆ with the voltage applied through the resistor R₅ and tends to pull down the voltage at the input of the inverter I₁. When the capacitor C₅ has been charged to the extent that the voltage thereacross is above the upper trip point of the inverter I₁, the output of the inverter I₁ goes low and the output of the inverter I₂ goes high. The high output of the inverter I₂ pulls up the input of the inverter I₁ through the resistor R₆. In summary, before closure of the reed switch RS₁ the output of the inverter I₂ and thereby the circuit 11 is low. At a certain time after closure of the reed switch RS₁ the output of the inverter I₂ snaps to a high value. The capacitors C₁, C₂, C₄ and C₅ are charged up to the voltage V of the battery 12, after which time current flow through the circuit 11 drops to substantially zero. The high input impedance of the snap action circuit 23 prevents discharge of the capacitors C₁, C₂, C₄ and C₅ therethrough and thereby current flow through the circuit 11.

Between the time t₁ and a time t₂ which may be much later, the shaft 21 rotates to a position at which the reed switch RS₁ opens. However, this has no effect on the circuit, even if both reed switches RS₁ and RS₂ are left open for a prolonged period of time.

At the time t₂, the reed switch RS₂ is closed by the magnet assembly 19, causing the capacitor C₂ to discharge through the reed switch RS₂ and resistor R₂. The peak current flow through the resistor R₂ is V/r₂, since the capacitor C₂ was chargedup to the voltage V. Since the capacitor C₁ was also charged up to V, closure of the reed switch RS₂ is equivalent to shorting the point 18 to the point 13. Thus, the capacitor C₁ discharges through the resistor R₁ and reed switch RS₂ with the peak value of current through the resistor R₁ being V/r₁. The combined current flow through the reed switch RS₂ is V/r₁ + V/r₂. Thus, the peak current flow through the reed switch RS₂ is the same as the peak current flow through the reed switch RS₁. However, the direction of current flow is just opposite as indicated in FIG. 2a.

As viewed in FIG. 2a, the contacts of the reed switch RS₂ bounce in the same manner as the contacts of the reed switch RS₁. However, due to the circuit time constant, the capacitor C₂ discharges, along with the capacitors C₁, C₄ and C₅ in a smooth manner as indicated in FIG. 2b. When the voltage across the capacitor C₅ drops below the lower trip point of the inverter I₁ the output of the inverter I₁ goes high and the output of the inverter I₂ goes low. The low output of the inverter I₂, which constitutes the output of the circuit 11, is applied to the input of the inverter I₁ as mentioned above and pulls the same down, thereby providing a hysterisis zone between the upper and lower trip points of the inverter I₁.

At a certain time after t₂, the reed switch RS₂ is opened by the magnet assembly 19 and the capacitor C₁ is charged up to V. However, there are no other changes in the circuit 11, even if both reed switches RS₁ and RS₂ are left open for a prolonged period of time since the capacitors C₂, C₄ and C₅ are discharged and disconnected from the point 13.

In summary, it will be seen that closure of the reed switch RS₂ causes the output of the inverter I₂ and thereby the circuit 11 to snap down to a low value. The output of the circuit 11 is a square pulse having a duration substantially equal to the length of time between closure of the reed switches RS₁ and RS₂. The rising edge of the pulse substantially corresponds to the time of closure of the reed switch RS₁. The falling edge of the pulse substantially corresponds to the time of closure of the reed switch RS₂. The pulse is completely clean and uneffected by bouncing of the contacts of the reed switches RS₁ and RS₂ and electrical noise introduced into the circuit 11 due to the action of the low-pass filter 22 and snap action circuit 23. Preferably, the low-pass filter 22 and snap action circuit 23 are provided at the remote utility station so as to best eliminate the effects of noise introduced into the connecting cables. Although the low-pass filter 22 is shown as having two stages, one or more than two stages can be provided. The low-pass filter 22 and snap action circuit 23 may be fabricated using CMOS integrated circuit technology, thus making them inexpensive, compact, light and reliable and also providing in-phase rejection (common-mode rejection) of noise.

Whereas the snap action circuit 23 provides one long square pulse for each revolution of the shaft 21, a modified snap action circuit 31 which is shown in FIG. 4 produces a short negative pulse each time the reed switch RS₁ or RS₂ is closed, or two pulses for each revolution of the shaft 21. In FIG. 4, like elements are designated by the same reference numerals as used in FIG. 3. In the snap action circuit 31, the resistor R₅ is omitted and the output of the low-pass filter 22 is connected directly to the input of the inverter I₁. A resistor R₇ is connected between the output of the inverter I₁ and the input of the inverter I₂. A capacitor C₉ is connected between the input of the inverter I₂ and the point 14. The input of the inverter I₁, output of the inverter I₁, input of the inverter I₂ and output of the inverter I₂ are designated as circuit points 32, 33, 34 and 36 respectively. An exclusive OR gate O₁ has inputs connected to outputs of the inverters I₁ and I₂ respectively. The output of the gate O₁ constitutes the output of the snap action circuit 31 and is designated as a circuit point 37. FIGS. 5a to 5e illustrate the voltages at the points 32, 33, 34, 36 and 37 as functions of time. The inverters I₁ and I₂ are assumed as each being provided with a hysterisis function.

At the time t₁ the voltage at the point 32 is low, causing the inverter I₁ to produce a high output and the inverter I₂ to produce a low output. Since the voltages at the inputs of the gate O₁ are dissimilar, the output of the gate O₁ is high.

After the reed switch RS₁ is closed and the capacitor C₅ is charged to the extent that the voltage at the point 32 reaches the upper trip point of the inverter I₁ as indicated by a line 41 in FIG. 5a, which occurs at a time t₃, the output of the inverter I₁ goes low. Since the outputs of the inverters I₁ and I₂ are both low, the gate O₁ produces a low output. The capacitor C₉ discharges through the resistor R₇ and inverter I₁ until the voltage at the point 34 drops below the lower trip point of the inverter I₂ which is indicated by a line 42 in FIG. 5c. This occurs at a time t₄. The output of the inverter I₂ goes high. As can be seen in FIG. 5a, at the time t₄, the high output of the inverter I₂ is summed with the voltage across the capacitor C₅ so that the voltage at the point 32 increases at a higher rate. Since the inputs of the gate O₁ are again dissimilar, the gate O₁ produces a high output. In summary, the exclusive OR gate O₁ produces a negative pulse between the times t₃ and t₄ shortly after the reed switch RS₁ is closed.

Opening of the reed switch RS₁ has no effect on the circuit 31. However, when the reed switch RS₂ is closed at time t₂, the voltage at the circuit point 32 decreases. When the voltage at the point 32 drops below the lower trip point of the inverter I₁ which is indicated by a line 43 in FIG. 5a, the output of the inverter I₁ goes high and the capacitor C₉ starts to be charged through the inverter I₁ and resistor R₇. This occurs at a time t₅. Since the inputs of the gate O₁ become similar, more specifically both high, the output of the gate O₁ again goes negative. When the capacitor C₉ is charged to the extent that the voltage at the point 34 exceeds the upper trip point of the inverter I₂ which is indicated by a line 44 in FIG. 5c, the output of the inverter I₂ goes low. This occurs at a time t₆. Since the inputs of the gate O₁ again become dissimilar, the gate O₁ produces a high output. At the time t₆, the low output of the inverter I₂ is summed with the voltage across the capacitor C₅ causing the voltage at the point 32 to drop at a faster rate. Thus, it will be seen that the gate O₁ produces a negative pulse between the times t₅ and t₆.

FIG. 6 shows a modified switch circuit of the invention which is designated as 51. Again, like elements are designated by the same reference numerals. The circuit 51 comprises fifth, sixth and ninth capacitors C₆, C₇ and C₈ which are connected between the capacitors C₂, C₄ and C₅ respectively and the point 13. This provides a balanced or symmetrical arrangement relative to the ends of the battery 12 which is desirable in certain applications. For instance, this serves to improve in-phase rejection of noise. The snap action circuit is modified to conform to this configuration and is designated as 52.

As a further point of difference the reed switches RS₁ and RS₂ are replaced by a single pole, double throw switch 53 which comprises a movable contact 53a connected to the point 16 and two fixed contacts 53b and 53c. The movable contact 53a is selectively engageable with the fixed contacts 53b and 53c which are connected to the points 17 and 18 respectively. The switch 53 is preferably operable by means of a cam (not shown) fixed to the shaft of the utility meter in such a manner as to alternately connect the point 16 to the points 17 and 18 during each revolution of the shaft.

Thus, it will be seen that the present invention provides a mechanical switch circuit comprising an improved contact conditioning means which is especially suited for applications involving remote reading of utility meters and the like. The circuit eliminates the effects of contact bouncing and electrical noise from the operation of the circuit. The circuit further provides an economical means of promoting and prolonging the effective engagement of the switch contacts by temporary partial fusion thereof. Further, the circuit is not adversely effected if the switches are left open for prolonged periods of time. Furthermore, no current path is completed as a principle when there is no change in the position of the contacts, thus reducing power consumption. Numerous modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof. 

What is claimed is:
 1. A switch circuit comprising:a D.C. power source having first and second ends; a first resistor, a second resistor and a first capacitor connected in series across the first and second ends of the power source in such a manner that the capacitor is connected between the first and second resistors; a second capacitor connected at a second end thereof to the second end of the power source; and a mechanical switch means for selectively connecting a first end of the second capacitor to first and second ends of the first capacitor which are connected to the first and second resistors respectively.
 2. A switch circuit as in claim 1, further comprising a third capacitor connected across the first and second ends of the power source.
 3. A switch circuit as in claim 1, further comprising a low-pass filter connected across the second capacitor.
 4. A switch circuit as in claim 3, in which the low-pass filter comprises a third resistor and a fourth capacitor connected in series in such a manner that the third resistor is connected to the first end of the second capacitor and the fourth capacitor is connected to the second end of the power source, the third resistor and the fourth capacitor constituting a first stage of the low-pass filter.
 5. A switch circuit as in claim 4, in which the low-pass filter further comprises a fourth resistor and a fifth capacitor connected in series in such a manner that the fourth resistor is connected through the third resistor to the first end of the second capacitor and the fifth capacitor is connected to the second end of the power source, the fourth resistor and the fifth capacitor constituting a second stage of the low-pass filter.
 6. A switch circuit as in claim 1, further comprising a snap action circuit having an input connected between the first end of the second capacitor and the second end of the power source.
 7. A switch circuit as in claim 6, in which the snap action circuit has a high input impedance.
 8. A switch circuit as in claim 6, in which the snap action circuit comprises a Schmitt trigger.
 9. A switch circuit as in claim 8, in which the Schmitt trigger comprises first and second cross-coupled inverters.
 10. A switch circuit as in claim 8, in which the Schmitt trigger further comprises an integrating circuit connected between an output of the first inverter and an input of the second inverter, the snap action circuit further comprising an exclusive OR gate having inputs connected to outputs of the first and second inverters.
 11. A switch circuit as in claim 1, in which the mechanical switch means comprises a first reed switch connected between the first end of the second capacitor and the first end of the first capacitor and a second reed switch connected between the first end of the second capacitor and the second end of the first capacitor.
 12. A switch circuit as in claim 1, further comprising a sixth capacitor connected between the first end of the power source and the first end of the second capacitor.
 13. A switch circuit as in claim 5, further comprising a seventh capacitor connected between the first end of the power source and a junction of the third resistor and fourth capacitor.
 14. A switch circuit as in claim 13, further comprising an eighth capacitor connected between the first end of the power source and a junction of the fourth resistor and the fifth capacitor. 